Refactor tests so that they fail when a dependency that should be installed is not installed

[freddyaboulton]

This came out of out Sept B sprint retro. Our CI silently failed to install Prophet and we didn't notice because the Prophet tests didn't fail!

The reason they didn't fail is because we have pytest.import_or_skip at the top of the prophet test file. This means that the prophet tests were being skipped in the git-test-prophet job!

We should refactor our tests so that they fail when a dependency that should be installed is missing.

One way to do this is to move away from pytest.import_or_skip and instead leverage the --has-minimal-dependencies custom pytest option in our conftest.py to skip tests.

This will make it so that tests that need our optional dependencies are skipped when we run our core dependencies tests. When we run our core + optional dependencies tests, no tests should be skipped from missing dependencies.

We can follow this guide from the pytest documentation so that we can skip tests based on command line values.